Why does `NH_3` readily form complexes but `NH_4^(+)` does not ?

Explanation

`NH_3` contains a lone pair of electrons which can co-ordinate with metal ion to form complex compound. However, in `NH_4^(+)` ion lone pair is bound to `H^(+)` andtherefore not available for bonding to metal ion. So `NH_4^(+)` does not form complex.